nextjs - 💡(How to fix) Fix Duplicate SWC transform for `use client` components when React Compiler is disabled [1 comments, 2 participants]

Link to the code that reproduces this issue

https://github.com/canyon-project/nextjs16-swc-duplicate-compilation

To Reproduce

Description

When using Next.js 16 with SWC and React Compiler disabled, use client components appear to be transformed twice by SWC.

The duplicate transformation produces identical AST output.

This behavior can be observed using a custom SWC plugin that writes intermediate stage output to disk.

Reproduction Repository

https://github.com/canyon-project/nextjs16-swc-duplicate-compilation.git

How to Observe

The reproduction includes a custom SWC plugin:

.swc-plugin-stage-output

This plugin writes SWC stage outputs to:

.swc-plugin-stage-output/

After running build:

pnpm build

Please inspect:

.swc-plugin-stage-output/

You will see:

• The same source file being transformed twice
• The generated AST output is identical
• The transform stage identifiers indicate two independent passes

Observed Behavior

• Only happens when:

  • Component contains "use client"
  • React Compiler is disabled

• Does NOT happen when React Compiler is enabled

• Both compilation passes produce identical AST output

• SWC plugin hooks are executed twice for the same file

Questions

1. Is this double transform expected?
2. Is this related to separate client/server graph compilation?
3. If intentional, is identical AST transformation unavoidable?
4. Could this be optimized to avoid redundant identical passes?

Why This Matters

In projects with many use client components:

• SWC plugin execution cost doubles
• Instrumentation cost doubles
• Build time and memory may increase

For tooling that relies on SWC transforms (e.g., instrumentation plugins), this introduces measurable overhead.

Technical Hypothesis (Optional)

It appears that:

• Next.js builds separate client and server graphs
• use client modules are processed independently in both graphs
• However, in the no-React-Compiler case, both passes generate identical AST

If this duplication is purely architectural (graph separation), perhaps memoization or transform result reuse could reduce redundant work.

Current vs. Expected behavior

Observed Behavior

• Only happens when:

  • Component contains "use client"
  • React Compiler is disabled

• Does NOT happen when React Compiler is enabled

• Both compilation passes produce identical AST output

• SWC plugin hooks are executed twice for the same file

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.0.0: Wed Sep 17 21:41:45 PDT 2025; root:xnu-12377.1.9~141/RELEASE_ARM64_T6000
  Available memory (MB): 16384
  Available CPU cores: 8
Binaries:
  Node: 25.2.1
  npm: 11.6.2
  Yarn: N/A
  pnpm: 10.13.0
Relevant Packages:
  next: 16.1.6 // Latest available version is detected (16.1.6).
  eslint-config-next: N/A
  react: 19.2.3
  react-dom: 19.2.3
  typescript: 5.9.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

SWC

Which stage(s) are affected? (Select all that apply)

next build (local)

Additional context

No response